The present invention relates to a method for selective rich/lean control of a combustion mixture using an electrochemical gas sensor according to the definition of the species in Claim 1.
In the course of fuel economy, internal combustion engines are preferably operated today using a lean combustion mixture. As a result, the nitrogen oxides NOx can no longer completely react in the catalytic exhaust-gas converter, since the necessary reducing components are no longer present in sufficient amounts. Because of this fact, so-called NOx adsorption catalysts are used that are capable of storing excess NOx. However, this presupposes that the catalyst is regenerated from time to time, as soon as the storage capacity of the NOx adsorption catalyst is exhausted. For this purpose, the combustion mixture is set to be rich for a short time, until the stored NOx is completely converted and a new storage phase can be introduced. To be able to successfully carry out this rich/lean control, both a gas sensor for determining the NOx, which detects the increase in the NOx concentration occurring at the end of the storage phase, and a gas sensor for determining the oxygen concentration, which analyzes the rich combustion exhaust with a particularly low oxygen content emerging from the catalytic converter at the end of the regeneration phase, are necessary in the exhaust-gas stream, in the flow direction, after the NOx adsorption catalyst.
An electrochemical gas sensor as described, for example, in Patent Application DE 199 12 102.8 can be used as the NOx gas sensor. It includes two chambers for the gas to be measured, each having a pump cell in different layer planes of a planar, oxygen ion-conducting, ceramic base material. Both pump cells include two electrodes deposited on a solid electrolyte. The gas to be measured flows via a first diffusion opening into the first measuring gas chamber where a first pump cell sets a constant low oxygen partial pressure by pumping oxygen in and out. With the aid of the electrical voltage (electromotive force) of a concentration cell (Nernst cell) also situated in the first measuring gas chamber, the oxygen partial pressure in the first measuring gas chamber is adjusted via the pump voltage of the pump cell. In a second measuring gas chamber, the NOx concentration in the measuring gas is determined in that the NOx contained in the measuring gas is decomposed on the surface of an electrode belonging to the second pump cell, and the oxygen resulting therefrom is pumped off together with the oxygen still remaining in the measuring gas.
An additional gas sensor, e.g. an oxygen sensor representing a lambda sensor, is needed to determine the oxygen concentration in the exhaust-gas stream. The disadvantage of this arrangement is the incorporation of two separate gas sensors, which entails an enormous cost disadvantage.
The method according to the present invention and having the characterizing features of Claim 1 has the advantage that a NOx gas sensor underlying the method, as described, for example, in Patent Application DE 199 12 102.8, is not only used to determine the NOx concentration in the measuring gas, but also to measure the oxygen concentration present there. This makes it unnecessary to install a separate oxygen sensor and makes possible the selective control of the combustion mixture using one and the same sensor.
According to one example embodiment of the present invention, a method for selective rich/lean control of a combustion mixture for an operation of an NOx adsorption catalyst, includes: storing NOx in a storage phase when the combustion mixture is set to be lean; converting the stored NOx in a regeneration phase when the combustion mixture is set to be rich; using an NOx concentration and an oxygen concentration in combustion exhaust gases in a flow direction behind the NOx adsorption catalyst for monitoring the storage phase and for monitoring the regeneration phase, respectively; determining the NOx concentration and the oxygen concentration by the same electrochemical gas sensor; and causing, using a measuring signal of the gas sensor, the storage phase to be initiated in response to a predetermined oxygen concentration not being attained, while a combustion mixture set to be rich is present.
The features set forth in the dependent claims make possible advantageous developments of the method recited in Claim 1. Furthermore, the dependent claims describe that, depending on the demand for precision of measurement, there is a plurality of different possibilities for operating the gas sensor for determining oxygen, and the method can, thus, be better adjusted to possible control engineering requirements.